1. Field of the Invention
The present invention relates to a fluid pressure operated torque amplifier (sometimes called "torque generator") and a vehicular power steering system incorporating such torque amplifier.
2. Description of the Prior Art
Generally, a fluid pressure operated torque amplifier has been used in a power steering system for a construction vehicle, an agricultural machine such as a rice transplanter, or a ship, and also in a gate or valve opening or closing device which needs large torque to operate it. Such a torque amplifier typically comprises a housing having a pressurized fluid inlet in communication with a source of pressurized fluid and a return fluid outlet in communication with a reservoir, a torque amplifier unit having an internally toothed member and an externally toothed member disposed within the internally toothed member for orbital motion therewith, the teeth of the internally and externally toothed members intermeshing to form a plurality of expanding and contracting volume chambers, a control valve assembly for controlling the fluid flow between the pressurized fluid inlet and the return fluid outlet, an input member operatively associated with the control valve assembly, and an output member for transmitting the power generated by the torque amplifier unit.
In the vehicular power steering system, such a fluid pressure operated torque amplifier is used as a steering actuator for wheels and their associated steering linkage. Disposed between the torque amplifier unit and the steering linkage, in a power transmitting relationship, is a reduction gear unit comprising typically an input gear and an output gear in meshing engagement therewith, which gear unit is positioned within a cavity in a transmission casing serving as a fluid reservoir. The input gear of the reduction gear unit has an input shaft extending upwardly through the transmission casing for coupling to the downwardly extending output shaft of the torque generator. The pressurized fluid inlet and the return fluid outlet of the torque amplifier are in communication with the cavity in the transmission casing by way of a pressurized fluid line connected to a pump and a separate return fluid line, respectively, so that when the control valve assembly is operated from a neutral position to a steering operating position, the pressurized fluid coming from the pump through the pressurized fluid line flows through the inlet port, the control valve assembly, the torque amplifier unit, the outlet port and the return line to the reservoir. Disposed between the pressurized fluid line and the return fluid line is a relief valve which acts to relieve the pressure depending upon steering conditions, for example, when the wheels become stuck on an obstacle.